There is known a mixed-reality (MR) technique of making an object (virtual object) rendered by computer graphics (CG) look as if it existed in the real world. The MR technique superimposes an image in a real space and an image in a virtual space generated by three-dimensionally modeled CG, aligns the images with each other, and displays them.
A mixed-reality presentation system (to be referred to as an “MR presentation system” hereinafter) using the MR technique is formed from the following apparatuses. That is, the system is formed from a real-image capturing apparatus (for example, a video camera) for capturing the real world, an image processing apparatus for generating a virtual space image observed from an image capturing position in the real world, and generating a mixed-reality space image (to be referred to as an “MR image” hereinafter) by compositing the images, and an image displaying apparatus for displaying the MR image. A personal computer (PC) or work station is generally used as an image processing apparatus.
As one of MR presentation systems, a system using a video see-through type head mounted display (to be referred to as “HMD” hereinafter) is known. This system captures an object by an image capturing unit whose image capturing direction is made to almost coincide with the line of sight from the pupil position of an HMD wearer (to be referred to as a “wearer” hereinafter), and presents, as an arbitrary stereoscopic image, an MR image generated using the captured image to the wearer in real time.
The HMD is required to provide a clear image closer to reality to the wearer in real time by improving the image capturing resolution and display resolution. To satisfy this requirement, it is necessary to increase the data amount of a captured image and display image, which is transmitted/received between the HMD and an HMD controller or between the HMD controller and the image processing apparatus.
A cable (to be referred to as a “metal cable” hereinafter) using metal wires as transmission media is often used to connect the HMD and the image processing apparatus. Wireless connection can be used instead of wired connection, as a matter of course. Since, however, the wireless communication band is narrow, the wireless connection is limited to an HMD which processes low-resolution images. Furthermore, to observe a video (CG and the like) from various positions or angles while the wearer wears the HMD, a relatively long cable, for example, a cable of about 10 meters is required. Since the cable length and the communication band have a trade-off relationship, if a long metal cable is used, a signal deteriorates to increase the occurrence probability of a bit error.
If a cable (to be referred to as an “optical cable” hereinafter) using optical fibers as transmission media is used, wide-band and long-distance transfer becomes possible and resistance against noise such as an electromagnetic wave also improves. However, it is necessary to adequately consider handling of the optical cable so as not to degrade a fitting portion (especially, optical fiber end faces) in a non-fitting state. Japanese Patent Laid-Open No. 2004-179733 (literature 1) proposes a cable in which a transmission module including a laser diode (LD) and LD driver and a control unit for controlling the amount of light emitted by the LD are incorporated in a connector, and a reception module including a photodiode and amplifier and a received light amount detection unit are incorporated in another connector. This cable also includes control signal lines and power supply lines between a transmission side and reception side, and is an optoelectronic composite cable obtained by compositing optical fibers and electrical signal metal wires.
Using an optoelectronic composite cable for connection between the HMD and the HMD controller or between the HMD controller and the image processing apparatus is highly advantageous since it is not necessary to mount a battery on the HMD. However, the input signal of the LD driver and the output signal of the amplifier in the optoelectronic composite cable are often differential signals, and the signals are in an AC coupling state. As a result, when the power supply is turned on, a signal having an indefinite frequency and the maximum amplitude (peak-to-peak amplitude) will be outputted in many cases. Consequently, in a state in which the reception function of an apparatus (for example, an HMD) for receiving a signal does not operate, that is, when a reception integrated circuit is in a non-operation state, a signal having an indefinite frequency may be input and cause a failure of the reception integrated circuit.
Light used for optical communication is a semiconductor laser beam of a short wavelength. When an optical cable is cut off for some reason, if optical output is continued and a person carelessly looks at a section, their eyes may be adversely affected. Therefore, it is necessary to stop optical output when an optical cable is cut off.
To solve these problems, an optical communication apparatus disclosed in Japanese Patent Laid-Open No. 2007-306213 (literature 2) starts to supply power to the photoelectric converter of an optical module upon detecting connection of the optical module and receiving a communication request from a host apparatus. In other words, it is confirmed whether optical communication is possible, and if communication is possible and a communication request is received, power supply to the photoelectric converter is started. If this method is applied to an HMD, a detection signal is transmitted even when the communication integrated circuit of a device like an HMD is in a non-operation state in order to detect the connection state of the device, and it is thus impossible to solve the problem that the communication integrated circuit may fail. In addition, since it is necessary to periodically transmit a detection signal to the device to monitor connection of the device, this method cannot be considered as an efficient method.
A technique disclosed in International Publication WO2012/105440 (literature 3) performs connection detection, and starts or interrupts power supply to a necessary photoelectric converter in accordance with the type of a connected apparatus. If this method is applied to an HMD, an optical signal is transmitted even when the communication integrated circuit of a device like an HMD is in a non-operation state in order to detect the connection state of the device, and it is thus impossible to solve the problem that the communication integrated circuit may fail.
Japanese Patent Laid-Open No. 2011-130297 (literature 4) discloses a method similar to that in literature 3, which performs connection detection, and starts or interrupts power supply to a necessary photoelectric converter in accordance with a combination of the type and state of a connected apparatus. That is, for the main purpose of saving power and preventing a transmission line from deteriorating, an optical output control circuit detects a power supply, ground, and information (connection detection, the type of a connected apparatus, a power on/off state) of connected apparatus, and starts or interrupts power supply to a photoelectric converter requiring communication. If a method of interrupting power supply to an unnecessary photoelectric converter is applied to an HMD, when power supply is restarted, it is necessary to set initial parameters and the like of the communication integrated circuit and the like, thereby taking the time to activate the apparatus. The method disclosed in literature 4 is effective when various kinds of host apparatuses and devices are variously combined. However, in an apparatus for which combinations of connections are limited, a connector with the large number of pins must be used for unnecessary control signals.
According to Japanese Patent Laid-Open No. 2001-160777 (literature 5), the current consumption of a device is detected to determine whether the device is in a communicable state. Since this method determines based on the current consumption of the device whether the device is in the communicable state, a complicated detection circuit considering a change in current consumption caused by temperature fluctuations and the like is required, thereby complicating the circuit.